[Qt] let OptionsModel::getProxySettings() directly query proxy

- as a proxy set via GUI can be overridden via -proxy, directly query the
  core to get active proxy
- give a warning, if active proxy is not SOCKS5 (needs to be SOCKS5 for
  the Qt networking code to work)
- also remove an obsolete connect() call from optionsdialog.cpp and a
  reference to Bitcoin-Qt (now just GUI)
Fix importprivkey / rescan
wallet: add interface for storing generic data on destinations
[Qt] Permanently store requested payments in wallet
[Qt] Add sorting feature to the requested payments table
doc: Build with qt5 is possible again

This reverts commit 62568f9a1e98065bd51069649f23a337eab89f94.
Also add some documentation for `--with-gui`.
Merge pull request #3564

54d2b91 doc: Build with qt5 is possible again (Wladimir J. van der Laan)
[Qt] Show and store message of normal bitcoin:URI
Merge pull request #3502

1f12844 Fix importprivkey / rescan (Cozz Lovan)
Merge pull request #3521

4d90102 [Qt] Add sorting feature to the requested payments table (Cozz Lovan)
8476d5d [Qt] Permanently store requested payments in wallet (Cozz Lovan)
b10e147 wallet: add interface for storing generic data on destinations (Wladimir J. van der Laan)
Merge pull request #3452

1ba3560 [Qt] let OptionsModel::getProxySettings() directly query proxy (Philip Kaufmann)
enable full GCC Stack-smashing protection for all OSes

- change our hardening options to use -fstack-protector-all even for
  Windows builds, as we recently switched to a newer compiler suite
- also removes an obsolete workaround for GCC 4.5
  (https://bugs.launchpad.net/ubuntu/+source/gcc-4.5/+bug/691722), which
  required to first set -fno-stack-protector, before -fstack-protector-all
[Qt] very minor style cleanups

- rebuilt some ui file layout to remove unreal values from the files
- remove an unneeded attribute from an ui file
- add / remove some spaces in files
build: Add -static for mingw builds

This avoids a dependency on libssp-0.dll when built with
-fstack-protector-all.
Merge pull request #1674

6ac0b3b build: Add -static for mingw builds (Wladimir J. van der Laan)
4e57e23 enable full GCC Stack-smashing protection for all OSes (Philip Kaufmann)
Merge pull request #3567

22a5120 [Qt] Show and store message of normal bitcoin:URI (Cozz Lovan)
Merge pull request #3568

4bee715 [Qt] very minor style cleanups (Philip Kaufmann)